1. Field of the Invention
This invention relates to a cord tensioner for a cord-guided covering, such as a roller blind, pleated blind or venetian blind or a combination of such blinds, for an architectural opening, such a vertical or a slanted window (e.g., a roof window).
2. Description of the Relevant Art
Cord tensioners have been conventionally used to keep guide cords of window coverings taut. Tensioned guide cords have been particularly important in coverings for slanted roof windows to prevent the fabrics and slats of the window coverings from sagging downwardly away from the windows. Cord tensioners have served to keep movable head rails, intermediate rails or bottom rails from sliding, under their own weight, down the guide cords. In this regard, the cord tensioners have maintained sufficient tension in the guide cords, so that there is enough friction between the guide cords and the movable rails to prevent the rails from sliding down the guide cords.
Cord tensioners for window coverings have traditionally been made from common tension springs as described in U.S. Pat. No. 4,733,711 and U.S. Pat. No. 4,557,309. However, such tension springs can be easily over stretched, particularly when movable rails of their window coverings are moved up or down abruptly. This is because the tension springs will bear any sudden force exerted on the guide cords before the movable rails slide along the guide cords. Any over stretching of the tension springs will reduce the tension in the guide cords and may make it necessary to retension the guide cords by either stretching the tension springs further or replacing them.
Tension springs have been protected against being over stretched by providing a cord between the two ends of each spring, so that the cord has a length less than the maximum allowable stretch of the spring along its axis. A problem with this solution is that the cord must always be too long for the spring in normal use, and the excess length of the cord can easily become snagged between the coils of the spring.